Lymphatic filariasis (LF) is a major cause of acute and chronic morbidity in the tropical and subtropical parts of the world. The availability of safe, single-dose, drug treatment regimens capable of suppressing microfilaremia to very low levels, along with improvements in techniques for diagnosing infection, has resulted in the targeting of this major mosquito-borne disease for global elimination. The Global Program to Eliminate Lymphatic Filariasis (GPELF) was launched in 2000 with the principal objective of breaking the cycles of transmission of Wuchereria bancrofti and Brugia spp. through the application of annual mass drug administrations (MDAs) to entire at-risk populations. Although significant progress in initiating MDA programs in endemic countries has been made, emerging challenges to this approach have raised questions regarding the effectiveness of using MDA alone to eliminate LF without the inclusion of supplementary vector control. Here, we review advances in knowledge of vector ecology, vector-parasite relationships, and both empirical and theoretical evidence regarding vector management to assess the feasibility and strategic value of including vector control in the GPELF initiative to achieve the global elimination of LF.

There is a consensus that malaria is a growing problem in African highlands. This is surprising because many parts of the highlands were considered too cold to support transmission. In this report, we examined how transmission of Plasmodium falciparum in six villages changed along an altitude transect in the Usambara Mountains, Tanzania, from 300 m to 1700 m. Routine entomological collections were made using spray catches and light traps for 15 mo. Direct estimates of entomological inoculation rates and indirect estimates of vectorial capacity suggested a >1000-fold reduction in transmission intensity between the holoendemic lowland and the hypoendemic highland plateau. Lowland transmission was perennial with a significant peak in the cool season after the long rains in May, when vectors densities were high. In the highlands, low temperatures prevented parasite development in mosquitoes during the cool season rains, and highland transmission was therefore limited to the warm dry season when vector densities were low. The primary effect of increasing altitude was a log-linear reduction in vector abundance and, to a lesser extent, a reduction in the proportion of infective mosquitoes. Highland malaria transmission was maintained at extraordinarily low vector densities. We discuss herein the implications of these findings for modeling malaria and suggest that process-based models of malaria transmission risk should be improved by considering the direct effect of temperature on vector densities. Our findings suggest that variation in the short rains in November and changes in agricultural practices are likely to be important generators of epidemics in the Usambaras.

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The impact of permethrin-impregnated bednets on resting and feeding behaviour of mosquito vectors of Wuchereria bancrofti, causing human lymphatic filariasis was studied in six pairs of villages (treated and untreated) before and after intervention. The study villages were in Kwale District, near the coast of Kenya, where Bancroftian filariasis is highly endemic, transmitted by a combination of both anopheline and culicine mosquito vectors. Mosquitoes were collected weekly in each village, indoors (using pyrethrum spray catches) and outdoors (using pit traps) during 3-4 months following the long rainy season. Of the filariasis vector species of mosquitoes collected in 1994 before intervention. 33.6% were members of the Anopheles gambiae complex, 30% were An. funestus and 36.4% were Culex quinquefasciatus. PCR analysis of the An. gambiae complex species collected in 1995 demonstrated that 98.5% were An. gambiae sensu stricto. 1% An. arabiensis and 0.5% An. merus. Introduction of impregnated bednets in 1995 significantly reduced the number of indoor-resting An. gambiae s.l. by 94.6% and An. funestus by 96.7%, but there was no change in the number of Cx quinquefasciatus collected indoors. The number of outdoor-resting An. gambiae s.l. was significantly reduced, whereas densities of An. funestus and Cx quinquefasciatus remained unaffected outdoors. ELISA analysis of mosquito bloodmeals showed a shift from human to animal feeding after the introduction of treated nets. The human blood index (HBI) for indoor resting Cx quinquefasciatus was reduced from 93.1% to 14.4%. Vector potential based on the HBI and mosquito density was estimated to be reduced by 99% for An. gambiae s.l., 98% for An. funestus and 97% for Cx quinquefasciatus and vectorial capacity would be suppressed even more by the impact on the vector survival rates (not measured). These results suggest that permethrin-impregnated bednets give effective personal protection against transmission of W. bancrofti by An. gambiae, An. funestus and Cx quinquefasciatus in East Africa.

BackgroundIn most countries of sub-Saharan Africa the control of lymphatic filariasis (LF) is based on annual mass drug administration (MDA) with a combination of ivermectin and albendazole, in order to interrupt transmission. Here we present the first detailed study on the effect of 3 repeated MDAs with this drug combination, as implemented by the Tanzanian National Lymphatic Filariasis Elimination Programme (NLFEP).Methodology/Principal FindingsInfection and transmission was monitored during a five-year period (one pre-intervention and four post-intervention years) in a highly endemic community (Kirare village) in north-eastern Tanzania. The vectors were Anopheles gambiae, An. funestus and Cx. quinquefasciatus. After start of intervention, human microfilaraemia initially decreased rapidly and statistically significant (prevalence by 21.2% and 40.4%, and mean intensity by 48.4% and 73.7%, compared to pre-treatment values after the first and second MDA, respectively), but thereafter the effect levelled off. The initial decrease in microfilaraemia led to significant decreases in vector infection and vector infectivity rates and thus to a considerable reduction in transmission (by 74.3% and 91.3% compared to pre-treatment level after first and second MDA, respectively). However, the decrease in infection and infectivity rates subsequently also levelled off, and low-level transmission was still noted after the third MDA. The MDAs had limited effect on circulating filarial antigens and antibody response to Bm14.Conclusion/SignificanceCritical issues that may potentially explain the observed waning effect of the MDAs in the later study period include the long intervals between MDAs and a lower than optimal treatment coverage. The findings highlight the importance of ongoing surveillance for monitoring the progress of LF control programmes, and it calls for more research into the long-term effect of repeated ivermectin/albendazole MDAs (including the significance of treatment intervals and compliance), in order to optimize efforts to control LF in sub-Saharan Africa.

BackgroundA dramatic decline in the incidence of malaria due to Plasmodium falciparum infection in coastal East Africa has recently been reported to be paralleled (or even preceded) by an equally dramatic decline in malaria vector density, despite absence of organized vector control. As part of investigations into possible causes for the change in vector population density, the present study analysed the Anopheles gambiae s.l. sibling species composition in north-eastern Tanzania.MethodsThe study was in two parts. The first compared current species complex composition in freshly caught An. gambiae s.l. complex from three villages to the composition reported from previous studies carried out 2–4 decades ago in the same villages. The second took advantage of a sample of archived dried An. gambiae s.l. complex specimens collected regularly from a fourth study village since 2005. Both fresh and archived dried specimens were identified to sibling species of the An. gambiae s.l. complex by PCR. The same specimens were moreover examined for Plasmodium falciparum and Wuchereria bancrofti infection by PCR.ResultsAs in earlier studies, An. gambiae s.s., Anopheles merus and Anopheles arabiensis were identified as sibling species found in the area. However, both study parts indicated a marked change in sibling species composition over time. From being by far the most abundant in the past An. gambiae s.s. was now the most rare, whereas An. arabiensis had changed from being the most rare to the most common. P. falciparum infection was rarely detected in the examined specimens (and only in An. arabiensis) whereas W. bancrofti infection was prevalent and detected in all three sibling species.ConclusionThe study indicates that a major shift in An. gambiae s.l. sibling species composition has taken place in the study area in recent years. Combined with the earlier reported decline in overall malaria vector density, the study suggests that this decline has been most marked for An. gambiae s.s., and least for An. arabiensis, leading to current predominance of the latter. Due to differences in biology and vectorial capacity of the An. gambiae s.l. complex the change in sibling species composition will have important implications for the epidemiology and control of malaria and lymphatic filariasis in the study area.

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